The present invention relates to a novel process for preparing polyolefins and more particularly to a process capable of initiating a polymerization reaction of olefins stably while preventing the formation of a sheet-like polymer in accordance with a vapor-phase polymerization method and thereby preparing polyolefins efficiently.
The vapor-phase polymerization method, which is one of polyolefin preparing methods, has recently been becoming most popular as an olefin preparing process because not only the removal of catalyst but also the removal and recovery of solvent are not necessary and hence the manufacturing cost is low. Usually, in a vapor-phase polymerization method of a fluidized bed type, the heat of polymerization generated in the polymerization reaction is removed using the sensible heat of circulating gas, so it is necessary that the polymer in the reactor be dispersed and fluidized uniformly and to a satisfactory extent. However, the polymer may be deposited on the reactor wall, thermometer, etc. due to static electricity generated by the friction between fluidized polymer particles or between polymer particles and the reactor wall, thus resulting in that the heat cannot be removed to a satisfactory extent and the polymer melts, causing the formation of agglomerated polymer or sheet-like polymer, which may cause the polymer withdrawing line to be closed and the operation stopped. Such a phenomenon is apt to occur when the interior of the reaction system is in an unsteady state at the time of start-up. As a method for preventing the generation of static electricity in the vapor phase reactor there has been proposed a method of adding, for example, water, alcohol, oxygen, nitrogen oxide or ketone (Japanese Patent Laid Open Nos. 230607/1989 and 145608/1990). However, all of those compounds have the drawback of deteriorating the polymerization activity.
It is the object of the present invention to remedy the above-mentioned drawbacks of the prior art, particularly provide a method for preventing the formation of a sheet-like polymer without deteriorating the polymerization activity.